National Geographic
Let’s start with the good news: If an asteroid capable of annihilating a city were heading toward Earth, and we had many years of advance warning, scientists already know how to prevent that disaster. NASA’s DART mission previously demonstrated that crashing a spacecraft into an asteroid on purpose could redirect a killer space rock away from the planet.
The bad news is that this technique won’t always work. So, then what?
For larger rocks or smaller ones detected too late, nuclear warheads, objects designed for mass destruction, may ironically turn out to be our salvation. And new research, using one of the most powerful radiation-generating machines ever built, suggests that blasting an asteroid with x-rays like those produced in a nuclear explosion can successfully knock even the larger, civilization-wrecking asteroids away from Earth.
The new study, published today in Nature Physics, suspended asteroid-like targets in a machine that bombarded them with pulses of radiation. Matter on the targets’ surface instantaneously vaporized, creating vaporous jets that essentially turns the targets into a temporary rocket, causing them to fly backwards.
“I knew right away that this was a huge success,” says Nathan Moore, a chemical engineer at Sandia National Laboratories in New Mexico and lead author of the new study. That rocket-like effect is exactly the sort of reaction planetary defenders would hope to see when trying to parry a real asteroid.
This experimental setup doesn’t provide a perfect simulation of an asteroid deflection mission using a nuclear explosion. But this scaled-down model offers a good way to test this technique without requiring a real-life deep space nuclear detonation, “so this is an exciting development,” says Angela Stickle, a hypervelocity impact physicist at the Johns Hopkins University Applied Physics Laboratory in Maryland who was not involved with the new work.
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(What’s the difference between an asteroid and a comet?)
How to nudge an asteroid
Let’s imagine astronomers spot a dangerously sized asteroid heading toward us.
If the space rock is small enough and detected at least a decade in advance, we could use a spacecraft called a kinetic impactor. That was the premise of DART: In September 2022, NASA intentionally crashed an uncrewed, semi-autonomous spacecraft the size of a van into a (harmless) 560-foot asteroid named Dimorphos at 14,000 miles per hour, significantly changing its orbit.
If we have fewer than about 10 years before impact, or that asteroid is large enough to devastate an entire country, then something like DART may not save us. With very large asteroids, even with plenty of advance notice, “a kinetic impactor, or even a fleet of kinetic impactors, may not be sufficient to prevent an Earth impact,” says Megan Bruck Syal, a planetary defense researcher at the Lawrence Livermore National Laboratory in California.
But a nuclear warhead might be able to deliver the formidable amount of energy and momentum required to save the planet.
“From the perspective of the physics of asteroid deflection, this makes it the only viable option” in these two scenarios, says Harrison Agrusa, a planetary scientist at the Côte d’Azur Observatory in France who was not involved with the new work.
With an extremely short warning time, space agencies may opt to disrupt the asteroid: they would blow it to smithereens, fragmenting it into tiny pieces that would mostly miss Earth or burn up inconsequentially in the atmosphere. Prior computer simulations have shown that a 330-foot-long asteroid (one capable of decimating a city) could be almost entirely vaporized with a one-megaton bomb if the space rock is nuked least two months before impact. But this is more of a Hail Mary approach, because it can risk turning a cannonball into a shotgun spray of asteroids.
Ideally, you want to deflect it. To accomplish this, an uncrewed spacecraft armed with a nuclear device would be commanded to park itself very close to the asteroid. Upon detonation, the bomb would release a burst of radiation, like x-rays, gamma rays, and neutrons. This radiation would careen into one side of the asteroid and get absorbed. That immediately shatters and vaporizes the rock, which gushes out into space and pushes the asteroid in the opposite direction.
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Something similar happened with the DART mission. When it hit Dimorphos—a weakly bound pile of rubble—the impact excavated and ejected so much debris that it gave Dimorphos a huge momentum boost and a significant deflection. It was as if 3.6 DART-like spacecraft hit the asteroid, meaning that bijou spacecraft punched well above its weight.
A nuclear bomb can provide an even more forceful wallop than something like DART. But using the most powerful nuke available is not necessarily the best plan, because you might accidentally break it apart. “Imagine that you slightly overestimated the energy needed for deflection, and now you have thousands of radioactive fragments falling on Earth,” says Sabina Raducan, a planetary scientist at the University of Bern in Switzerland who was not involved with the new work.
Conducting a planetary defense test in space with a nuclear device is unlikely to happen: A malfunctioning launch could spray radioactive material in-atmosphere, and any nation seeking to put nuclear warheads in space for any reason would stoke unprecedented political tensions.
Fortunately, data from nuclear weapons tests, high-energy experiment facilities (such as the National Ignition Facility at Lawrence Livermore National Laboratory), and cutting-edge computer simulations, strongly suggest that a finely tuned nuclear bomb deflection campaign “can be very effective at preventing Earth impacts,” says Bruck Syal, who was not involved with the new work.
A team of researchers wanted to test this theory. And to find out, they let tiny asteroids battle it out with faux nuclear explosions.
(These five asteroids post the greatest risk to Earth.)
Desktop nuclear blasts
The researchers turned to a device called the Z Machine at Sandia National Laboratories. This contraption uses intense electromagnetic fields to generate high temperatures, high pressures, and potent eruptions of x-rays. It’s so powerful that it can easily melt diamonds.
For their nuke deflection model, scientists targeted two minerals that show up in space rocks: a fingernail sized piece of quartz and a glassy piece of fused silica. At one end of the machine the targets were suspended in a vacuum, while at the other, a pocket of argon gas was subjected to an intense burst of electricity.
The argon imploded, transformed into a superhot, electrically charged gas called plasma, which emanated a torrent of x-rays at the targets—simulating a nuclear explosion in space. The team watched as the targets’ surfaces vaporized, creating supersonic jets that pushed the solid targets back at speeds of around 160 miles per hour.
Scaling their results up to genuine space rocks, the team estimate that even an asteroid 2.5 miles long could be gradually deflected away from Earth, given several years of warning.
Using ballistics in the laboratory to refine models of DART-like kinetic impactors is relatively routine work. But this experimental setup provides a novel way to test X-ray asteroid deflection techniques. “The authors have demonstrated some real innovation here,” says Patrick King, a physicist at the Johns Hopkins University Applied Physics Laboratory who was not involved with the new work.
The Z Machine setup has its limitations. The miniscule targets oversimplify what asteroids are truly like; the complex geologic composition and widely differing internal structures of real asteroids could affect the outcome of any planetary defense technique. “I’d be very interested to see how multi-mineral materials, like rocks or meteorites, behave,” Stickle says.
Questions also remain about whether nuclear devices can deflect asteroids with enough precision. And in any real-life asteroid emergency, there will always be a concern than the projectile could be inadvertently fragmented. Overall, though, the study offers welcome news for planetary defense. “I think it has been robustly shown, both in the lab and in computer simulations, that a nuclear device could deflect an asteroid,” Agrusa says.
“This isn’t to say that [nuclear devices] are always the answer,” says King. Using a nuclear device in any situation, including for planetary defense purposes, is fraught with danger. “Choosing to employ a [nuclear device] is a serious and potentially grave decision.” But ultimately, this study adds to a growing pile of evidence that a nuclear explosion can be used to save the world, particularly if we’re short on time.
“It’s reassuring that large asteroids don’t impact the earth very often,” says Moore. “It’s even more reassuring to know that now we have a way to prepare for that form of natural disaster.”
